mlxsw: spectrum: PTP: Disable BH when working with PHC

Up until now, the PTP hardware clock code was only invoked in the process
context (SYS_clock_adjtime -> do_clock_adjtime -> k_clock::clock_adj ->
pc_clock_adjtime -> posix_clock_operations::clock_adjtime ->
ptp_clock_info::adjtime -> mlxsw_spectrum).

In order to enable HW timestamping, which is tied into trap handling, it
will be necessary to take the clock lock from the PCI queue handler
tasklets as well.

Therefore use the _bh variants when handling the clock lock. Incidentally,
Documentation/ptp/ptp.txt recommends _irqsave variants, but that's
unnecessarily strong for our needs.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>